Loudspeaker device

ABSTRACT

A loudspeaker device comprising a power amplifier ( 13 ) to which an input signal is delivered via a subtracter ( 6 ), a speaker unit ( 1 ) for reproducing output signals delivered from the power amplifier ( 13 ), an acoustic pipe ( 2 ) coupled in front of the speaker unit for guiding the sound waves, a microphone ( 4 ) for detecting an acoustic outputs radiated from the speaker unit, a microphone amplifier ( 5 ) for amplifying an acoustic output signals detected by the microphone, and a negative feedback circuit. Output signals of the microphone amplifier are delivered to the subtracter, at the same time the same output signal is connected to a high-pass filter ( 7 ) to be delivered to the subtracter. Thus the negative feedback circuit is formed to suppress peaks and dips for providing a flattened sound pressure frequency characteristic.

FIELD OF THE INVENTION

The present invention relates to a loudspeaker (speaker) device for usein various acoustic apparatus and television receivers; morespecifically, a speaker unit coupled with an acoustic pipe disposed infront of the speaker, wherein the acoustic pipe has a microphone fordetecting sounds reproduced by the speaker unit, and sounds from thespeaker unit are corrected in accordance with the signals detected bythe microphone.

BACKGROUND OF THE INVENTION

A conventional speaker device of the same type is described withreference to FIG. 7, FIG. 8 (A) and FIG. 8 (B). FIG. 7 is a blockdiagram, FIG. 8 (A) shows microphone output signals, FIG. 8 (B) showssound output characteristics of the conventional speaker device, where,curve “a” shows a sound pressure characteristic, and curve “b” shows aphase characteristic.

Referring to FIG. 7, a speaker unit 1 generates sound waves, and thespeaker unit 1 is coupled with an acoustic pipe 2. At both sides of theacoustic pipe 2 are sound absorbing material (not shown) provided forsuppressing resonance. Inside the acoustic pipe 2, a microphone 4 isprovided close to the speaker unit 1 for detecting sound output signals.

When a signal is delivered to the speaker unit 1 via a subtracter 6 anda power amplifier 3, the speaker unit 1 radiates acoustic output, whichis radiated from the opening through the acoustic pipe 2. The standingwave due to the length of the acoustic pipe 2 and the one generatedwithin the acoustic pipe 2 causes a speaker device to reproduce soundshaving steep peaks and dips in the sound pressure frequencycharacteristic. In order to prevent this, a sound absorbing material isemployed to suppress the standing waves. However, the sound absorbingmaterial is not effective enough to suppress the standing wavescompletely. So, a microphone 4 detects the remaining standing wave andfeeds it back to the subtracter 6 via a microphone amplifier 5. Thus,the standing wave in acoustic pipe 2 is suppressed, and the reproducedsounds with a flat sound pressure frequency characteristic are obtained.

An acoustic pipe coupled in the front of a speaker unit is known toproduce a resonance in the pipe; the resonance frequency f generated isrepresented by the formula below:f=(n+1)C/4L

where; f: pipe resonance frequency, n: the n-th resonance, C: soundvelocity, L: length of the pipe.

In the above configured speaker device, when a primary resonance (n=1)due to the pipe length is corrected by means of the phase differencebetween the electrical input signal delivered to the speaker unit 1 andthe sound output signal radiated from the speaker unit 1, the resonancecomponent shifts and appears as a peak in the sound outputcharacteristic after the correction. So, it has been difficult toflatten the sound output characteristic. Furthermore, since the feedbackis performed for an entire frequency range from a low frequencycomponent to a high frequency component, it is impossible to control acertain desired frequency component.

The relationship between the input and the output is shown below:V out/V in=A/(1+A·T(S))

where; V out: output voltage, V in: input voltage, A: totalamplification by amplifiers, T (S) transfer function.

Assuming that the microphone 4 has an approximately flat characteristicand the T (S) is substantially equal to the transfer function of thespeaker unit 1, the T (S) becomes minus 1 as a result of phase shiftcaused by the speaker unit 1 and the second, or the third, piperesonance of acoustic pipe 2.

Namely, in some cases the denominator becomes 0 to be ready for makingoscillation. This makes it difficult to apply too many feedbacks takingan oscillation margin into consideration, and to effectively control alow frequency region and a pipe resonance.

The present invention addresses the above problems and aims to provide aspeaker device that has stable characteristics.

SUMMARY OF THE INVENTION

A speaker device of the present invention comprises a power amplifierwhich receives input signal via subtracter, a speaker unit forreproducing output signal of the power amplifier, an acoustic pipecoupled to the speaker unit in the front for guiding sound wavesgenerated by the speaker unit, a microphone for detecting acousticoutputs radiated from the speaker unit, and a microphone amplifier foramplifying acoustic output signals detected by the microphone. In aspeaker device having the above-described configuration, output signalsof the microphone amplifier are delivered to the subtracter, and, at thesame time, output signals of the above microphone amplifier aredelivered via a high-pass filter also to the substracter to form anegative feedback circuit in order to suppress peaks and dips in thesound pressure frequency characteristics. Thus, the speaker device isprovided with stable characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a speaker device in accordance with anexemplary embodiment of the present invention.

FIG. 2 shows an acoustic output characteristic.

FIG. 3 shows a block diagram of a speaker device in accordance withanother embodiment of the present invention.

FIG. 4 (A) shows a microphone output signal characteristic in accordancewith another embodiment of the present invention.

FIG. 4 (B) shows an acoustic output characteristic in accordance withanother embodiment of the present invention.

FIG. 5 shows a block diagram of a speaker device in accordance withstill another embodiment of the present invention.

FIG. 6 (A) shows a microphone output signal characteristic in accordancewith still another embodiment of the present invention.

FIG. 6 (B) shows a sound output characteristic in accordance with stillanother embodiment of the present invention.

FIG. 7 shows a block diagram.

FIG. 8 (A) shows a microphone output signal characteristic.

FIG. 8 (B) shows sound output characteristics.

DETAILED DESCRIPTION OF THE INVENTION

Now in the following, exemplary embodiments of the present invention aredescribed referring to the drawings FIG. 1 through FIG. 6 (B).

Those components having the same functions as those of the conventionaltechnology are represented by the same reference numerals used fordescribing the conventional speaker device.

First Embodiment

FIG. 1 shows a block diagram of an acoustic circuit in accordance with afirst exemplary embodiment of the present invention. FIG. 2 is a soundoutput characteristic chart; where, curve “a” shows the sound pressurecharacteristic, while curve “b” shows the phase characteristic.Initially, the overall structure of the speaker device is describedreferring to FIG. 1.

Referring to FIG. 1, in front of a speaker unit 1 an acoustic pipe 2 iscoupled to the speaker unit 1, and a microphone 4 is mounted within theacoustic pipe 2. Sound waves radiated from the speaker unit 1 aredetected by the microphone 4 within the acoustic pipe 2. The detectedsignals are delivered to a subtracter 6 via a high-pass filter 7, and atthe same time, the signals detected by the microphone 4 are inputdirectly to the subtracter 6 to be mixed with input signals coming fromoutside in order to correct the input signals. The corrected signals areamplified at a power amplifier 13 and delivered to the speaker unit 1.

The acoustic pipe 2 is disposed in front of a speaker box (not shown) inwhich the speaker unit 1 is mounted, and sound waves are guided by theacoustic pipe 2 to be radiated from a narrow slit opening having arectangular shape. The microphone 4 detects resonance in the acousticpipe 2, and feeds an acoustic output signals thus detected back to thesubtracter 6 via a secondary high-pass (−12 dB/oct) filter 7. At thesame time, the acoustic output signals are fed back directly to thesubtracter 6.

FIG. 2 shows a sound output characteristic. As compared with FIG. 8 (B),which shows the conventional counterpart, the peak due to a shift ofresonance component caused by phase shift is not seen in FIG. 2.Frequency characteristics of a speaker device in the present embodimenthave been flattened, without having a shift of the resonance component.

As described above, the resonance in the acoustic pipe 2 is detected bymicrophone 4, and the acoustic output signals thus detected aredelivered via the secondary high-pass filter 7 (−12 dB/oct) to thesubtracter 6 as the feedback. At the same time, the signals detected bythe microphone 4 are also delivered directly to the subtracter 6.Further, a cutoff frequency of secondary high-pass filter 7 (−12 dB/oct)is set to match with the resonance frequency of the pipe. By so doing,the phase correction is performed and a superior speaker device isprovided.

Second Embodiment

FIG. 3 shows a block diagram of a sound circuit in accordance with asecond exemplary embodiment of the present invention. FIG. 4 (A) shows amicrophone output signal characteristic, FIG. 4 (B) shows acousticoutput characteristics, where, curve “a” shows a sound pressurecharacteristic, while curve “b” shows a phase characteristic. Adifference compared to the first embodiment is that a negative feedbackcircuit in the present embodiment is formed by delivering the acousticoutput signals detected by microphone 4 to the subtracter 6 via a coupleof high-pass filters 7 and 8 connected in parallel. The filter 7 is thesecondary high-pass filter (−12 dB/oct), while the filter 8 is theprimary high-pass filter (6 dB/oct).

According to FIG. 8 (A), which shows the frequency characteristic of themicrophone signal of the conventional device, the feedback is performedcovering even the low frequency region components, which means the lowfrequency region components are enhanced. In the present embodiment,however, the level of low frequency region components is lowered asshown in the characteristic chart, as is shown in FIG. 4 (A). This meansthat the low frequency region components are not enhanced in the presentembodiment.

When the output sound characteristics of the present embodiment (FIG. 4(B)) are compared with those of the conventional device (FIG. 8 (B)), itis seen that no enhancement is given to the low frequency characteristicin the present embodiment, whereas the characteristic of theconventional device has been enhanced.

As described above, the negative feedback circuit in the presentembodiment is provided with a couple of primary and secondary high-passfilters 8 and 7 connected in parallel, and output signals of themicrophone amplifier 5 are connected thereto to be delivered to thesubtracter 6. The circuit can feed back those microphone output signalsonly in the vicinity of the resonance frequency component, so, theenhancement of the low frequency region components is suppressed. Thus,the output frequency characteristics can be flattened and corrected, anda speaker device of superior sound characteristics is provided.

Third Embodiment

FIG. 5 shows a block diagram of a sound circuit in accordance with athird exemplary embodiment of the present invention. FIG. 6 (A) showsthe microphone output signal characteristics, FIG. 6 (B) shows the soundoutput characteristics, where, curve “a” shows the sound pressurecharacteristic, while curve “b” shows the phase characteristic. Adifference compared to the first embodiment is that a negative feedbackcircuit in the present embodiment is formed of a couple of filters 7 and9. A secondary high-pass filter 7 for processing the output signaldetected by the microphone 4 and delivering to the subtracter 6, and alow-pass filter 9 of −12 dB/oct, or −6 dB/oct, for processing the outputsignal detected by the microphone 4 and delivering to the subtracter 6.

The low-pass filter 9 can take out only the low frequency regioncomponents for phase correction. Thus, the output sound characteristiccan be corrected for the low frequency region components alone. Thesecondary high-pass filter 7 can correct the acoustic pipe resonanceindependently and arbitrarily. In this way, the output frequencycharacteristics can be flattened and corrected easily and arbitrarily,and a speaker device of superior sound characteristics is provided inaccordance with the present embodiment.

When the sound output characteristics of the present embodiment FIG. 6(B) are compared with those of the conventional device FIG. 8 (B), itmay be clearly understood that the low frequency region characteristicand the pipe resonance can be controlled simultaneously, which allowsthe sound output characteristics to be controlled arbitrary.

Although the descriptions in the above embodiments have been based onthe high-pass filters for controlling the secondary resonance in thepipe, the n-th resonance (n being a positive integer) of the pipe can ofcourse be controlled in accordance with the present invention.

Furthermore, the present invention can be applied also to those speakerdevices whose back cover for coupling with the acoustic pipe is a bassreflective type, or having no back cover at all.

INDUSTRIAL APPLICABILITY

The speaker devices of the present invention comprise a power amplifierwhich receives an input signal via a subtracter, a speaker unit forreproducing an output signal of the power amplifier, an acoustic pipecoupled in a front of the speaker unit for guiding sound waves from thespeaker unit, a microphone for detecting acoustic outputs radiated fromthe speaker unit, and a microphone amplifier for amplifying the soundoutput signals detected by the microphone. The speaker device, which hasa negative feedback circuit formed by direct connection of the outputsignal of the microphone amplifier to the subtracter and by connectionof the above output signal of the microphone amplifier via a high-passfilter to the subtracter, makes the phase correction and suppresses thepeak due to a shift of resonance frequency component, and the shift iscaused by a phase change. The direct feedback of microphone outputsignal enables the low frequency region components to be enhanced. Theoutput frequency characteristics are thus flattened and the lowfrequency sound reproduction is improved so as to provide a speakerdevice of superior sound characteristics.

In the feedback circuit where a secondary high-pass filter is used forthe high-pass filter, remarkable effects are provided by matching thecutoff frequency with the resonance frequency.

In a speaker device comprising a power amplifier that receives an inputsignal via a subtracter, a speaker unit for reproducing an output signalfrom the power amplifier, an acoustic pipe coupled to a front of thespeaker unit for guiding sound waves, a microphone for detectingacoustic outputs radiated from the speaker unit, and a microphoneamplifier for amplifying acoustic output signals detected by themicrophone, and a negative feedback circuit formed of a couple ofprimary and secondary high-pass filters disposed in parallel forconnecting the output signals from the microphone amplifier to thesubtracter, the microphone output signals can be fed back only in thevicinity of the resonance frequency component. As a result, anenhancement in the low frequency region components can be suppressed.Thus, the output frequency characteristics can be easily flattened andcorrected for providing a speaker device of superior soundcharacteristics.

In a speaker device comprising a power amplifier that receives an inputsignal via a subtracter, a speaker unit for reproducing an output signalof the power amplifier, an acoustic pipe coupled in a front of thespeaker unit for guiding sound waves, a microphone for detecting soundoutputs radiated from the speaker unit, and a microphone amplifier foramplifying the sound output signals detected by the microphone, and anegative feedback circuit formed of a secondary high-pass filter and aprimary, or a secondary, low-pass filter disposed in parallel to beconnected to the subtracter, the output signals of microphone amplifiercan be fed back only in the low frequency component and that at thevicinity of resonance frequency component. The low-pass filter canperform a phase correction for the low frequency region component and acontrol of low frequency region component. Furthermore, any desiredfrequency components can be controlled independently, and the resonancefrequency component can be controlled while enhancing or suppressingreproduction of the low frequency region. In this way, the outputfrequency characteristics can be flattened and easily corrected, and aspeaker device of superior sound characteristics can be provided.

1. A loudspeaker device comprising: a power amplifier for receiving aninput signal via a subtracter; a speaker unit for reproducing an outputsignal of said power amplifier; an acoustic pipe coupled in front ofsaid speaker unit for guiding sound waves reproduced by said speakerunit; a microphone for detecting acoustic outputs radiated from saidspeaker unit; a microphone amplifier for amplifying an acoustic outputsignal detected by said microphone; and a negative feedback circuit,wherein said negative feedback circuit is formed by inputting an addedsignal to said subtracter, the added signal being an addition of a firstoutput signal delivered from said microphone amplifier and a secondoutput signal delivered from said microphone and passed through saidhigh-pass filter, and wherein a cutoff frequency of said high-passfilter is matched with a resonance frequency of said acoustic pipe.
 2. Aloudspeaker device comprising: a power amplifier for receiving an inputsignal via a subtracter; a speaker unit for reproducing an output signalof said power amplifier; an acoustic pipe coupled in front of saidspeaker unit for guiding sound waves reproduced by said speaker unit; amicrophone for detecting acoustic outputs radiated from said speakerunit; a microphone amplifier for amplifying an acoustic output signaldetected by said microphone; and a negative feedback circuit, whereinsaid negative feedback circuit is formed by inputting an added signal tosaid subtracter, the added signal being an addition of a first outputsignal delivered from said microphone amplifier and passed through a −12dB/oct. high-pass filter and a second output signal delivered from saidmicrophone and passed through a −6 dB/oct. high-pass filter, and whereina cutoff frequency of said −12 dB/oct high-pass filter is matched with aresonance frequency of said acoustic pipe.
 3. A loudspeaker devicecomprising: a power amplifier for receiving an input signal via asubtracter; a speaker unit for reproducing an output signal of saidpower amplifier; an acoustic pipe coupled in front of said speaker unitfor guiding the sound waves; a microphone for detecting acoustic outputsradiated from said speaker unit; a microphone amplifier for amplifyingan acoustic output signal detected by said microphone; and a negativefeedback circuit, wherein said negative feedback circuit is formed byinputting an added signal to said subtracter, the added signal being anaddition of a first output signal delivered from said microphoneamplifier and passed through a −12 dB/oct. high-pass filter and a secondoutput signal delivered from said microphone and passed through one of a−6 dB/oct. low-pass filter and a −12 dB/oct. low-pass filter, andwherein a cutoff frequency of said −12 dB/oct. high-pass filter ismatched with a resonance frequency of said acoustic pipe.